Method of making cold cathodes for vacuum tubes and article



D. w. MAYER 3,041,210 METHOD OF MAKING com: CATHODES FOR VACUUM TUBES AND ARTICLE June 26, 1962 Filed Dec. 2, 1959 IIIHHH INVENTOR DOA/44D M MA YEA QM, MW. Luau;

ATTORNEYS United States Patent )fiice METHOD OF MAKING CDLD CATHUDES FQR VACUUM TUBES AND ARTICLE Donald W.,Mayer, Colonia, N..I., assignor to Tung-5ol Electric Inc, a corporation of Delaware Filed Dec. 2, 1959, Ser. No. 856,851 11 Claims. (Cl. 117-.222)

The present invention relates to vacuum tube devices and more particularly to cold cathodes for use in such devices. Dobischek et a1. Patent 2,802,127 describes .a cathode coating of magnesium oxide in relatively porous'form that has the. characteristic of sustained electron emission once electron emission is initiated. Prior "to the present invention magnesium oxide, with or without traces of other materials, was believed unique in having the characteristic of sustained electron emission of the type above indicated and that it Was essential for "the cathodes of cold cathode vacuum tube devices to be provided with a magnesium oxide coating. In preparing a cathode having a magnesium coating, a metal sleeve, of nickel or the like, is first coated with the composition andthen after mounting within an envelope and prior to final exhaustion of the envelope, the coated sleeve is processed either in oxygen or in air to enhance or develop its characteristic of self-sustained electron emission.

I have found that cathode coatings having the characteristic of sustained primary electron emission can be prepared from oxides other than magnesium oxide, specifically from aluminum oxide, from beryllium oxide,

from boron sesquioxide and from mixtures'of two or cathode coatings having high electron emission may be prepared from mixtures of magnesium oxide with one or more of the above identified oxides.

In the preparation of cathode coatings containing no magnesium oxide, or a minor proportion thereof, I have found that a departure must be made from the procedure heretofore employed in preparing magnesium oxide coatings. Instead of processing the coated cathode in oxygen or air, the processing must be carried out in carbon dioxide, at a pressure of from 50 to about 100 mm. For example, a magnesium oxide coating admixed with a trace of aluminum oxide may be processed in air but when the amount of aluminum oxide in the composition is two or more times the amount of magnesium oxide the processing shouldbe carried out in carbon di oxide. An all aluminum oxide coating must be processed in carbon dioxide as must a beryllium oxide coating. The same is true of a coating of boron sesquioxide.

For a better understanding of the invention reference may be had to the accompanying drawing of which the single FIGURE is a side view partly broken away of a vacuum tube device employing the new cold cathode of the invention.

The vacuum tube device shown in the drawing is representative of any one of a number of devices that employ the new cathode. The particular device illustrated comprises an envelope 2 of glass or the like having mounted therewithin a cold cathode indicated generally at 4, a sustaining electrode 6, a control grid 8 and an anode 10.

The electrodes are mounted within the envelope on a suitable press 12 through which leads to conventional pin terminals 14 are sealed. The cathode 4 comprises a metal sleeve 16 of nickel or the like having a porous sponge-like oxide coating 18 thereon. Preferably within the sleeve 16 is a filamentary heater 20 used only during the initial Patented June 26, 1962 processing of the tube, the filament conveniently being a conventional cathode heater adapted for energization from a 6 or 12 volt source of energy. Preferably also within the envelope 2 and shielded from the cathode coating 18 by the posts supporting the sustaining and control grids, is a starter filament 22 for initiation of electron emission from the coating.

In operation of the above described device, potentials above that of the cathode are impressed upon the sustaining grid and anode, a load circuit is connected to the anode and control potentials are applied to the control grid. Electron emission'from the cathode coating is initiated by energization of the filament 22. After initiation the coating will continue to emit a copious supply of electrons so long as the sustaining grid is maintained at a suitable potential, say 200 volts, above that of the cathode sleeve.

The coating should be in porous sponge-like form in order to provide the avalanche effect during electron emission and thereby obtain substantial current per unit area of the coating. Irrespective of the particular oxide or mixture of oxides employed for the coating, the

sponge-like porous characteristic thereof may be obtained The so coated sleeve is then mounted within the envelope 2 together with the other electrodes.

The tube is then baked out in vacuum at about 300 C., then air or carbon dioxide, depending upon the coating composition, is admitted to a pressure of from 50 to 100 mm. and the cathode is heated to 900 C.

. in such atmosphere. The tube is then rebaked at 300 C.

in vacuum, following which all of the electrodes are heated to 700 C. for ten minutes. The electrodes, other than the cathode are then cooled while the cathode is heated to 850 C. for two or three minutes. The tube is then sealed oil and ready for use after preliminary aging. The above described process is substantially that which has been found most efi'ective in the preparation of magnesium Oxide coatings except that onlyair at mm. or oxygen at 20 mm. pressure has been used in processing magnesium oxide coatings.

The following examples are indicative of the materials that may be employed for the coating and of the particular atmosphere in which such coatings should be processed.

Example 1 The coating is 100% aluminum oxide. Processing is effected at a pressure of 100 mm. of carbon dioxide.

Example 2 3 Example 5 The coating is boron sesquioxide. The processing is done in carbon dioxide at a pressure of about 100 mm.

Example6 The coating is a mixture of aluminum oxide and boron sesquioxide. The processing is done in carbon dioxide at a pressure of about 100 mm.

Example 7 Example 8 The coating is a mixture of any two or more of the oxides of magnesium, beryllium, aluminum and boron in which the proportion by weight of magnesium oxide, if present, is from zero to less than one-half of the mixture. The processing is done in carbon dioxide at 100 mm. pressure.

Of the various above mentioned coatings of the examples the beryllium oxide coating, at the present time, has been found to have superior electron emission characteristics. Because of the health hazards involved in the use of berylium oxide, particularly if applied by spray coating, the beryllium oxide coating of Example 2 was prepared by reduction of a beryllium carbonate coating after the cathode was mounted in the tube.

Heating of the cathode in vacuum at 1100 C. reduced the carbonate to oxide. The coating described in each of the examples is capable of self-sustained electron emission of at least 3.3 milliamperes per square centimeter of the surface area of the coating. Each of the above described new coating compositions, when suitably processed, is at least equal, in sustained electron emission properties, to the prior art pure magnesium oxide coating. The coating compositions of Examples 2, 3, 4 and 7 are definitely superior to the prior art pure magnesium oxide coating.

From the foregoing description it will be apparent that the invention comprises not only the discovery that certain oxides other thanmagnesium oxide have the characteristics of self-sustained electron emission but also that the atmosphere in which such coatings are processed is of controlling importance. The processing in carbon dioxide is largely responsible for making possible the new coatings described herein. Although only three specific oxides, in addition to magnesium oxide, and mixtures thereof, have been specifically mentioned it is believed that any oxide or combination of oxides Which does not disintegrate at the processing temperature could be employed as a coating for cold cathodes provided the proper atmosphere were employed during the processing.

The following is claimed:

1. In the process of preparing a cathode coating having the characteristic of self-sustained primary electron emission in vacuum, the step comprising heating a cathode having a coating comprising less than 50% magnesium oxide admixed with an oxide of a metal selected from the class consisting of aluminum, beryllium and boron and mixtures thereof, in carbon dioxide only and ate pressure from 50 to mm.

2. A cathode made by the process of claim 1.

3. The cathode according to claim 2 wherein said coating contains aluminum oxide.

4. The cathode according to claim 3 wherein said coating also contains magnesium oxide.

5. The cathode according to claim 4 wherein the coating comprises a major proportion by weight of aluminum oxide.

6. The cathode according to claim 2 wherein said coating contains boron sesquioxide.

7. The cathode according to claim '6 wherein said coating also contains aluminum oxide.

8. The cathode according to claim 2 wherein said coating contains beryllium oxide.

9. In the process of preparing a cathode coatinghaving the characteristic of self-sustained primary electron emission in vacuum, the step comprising heating in carbon dioxide only and at a pressure of from 50 to 100 mm. a cathode having a coating thereon comprising at least 50% aluminum oxide and the remainder an oxide selected from the group consisting of magnesium oxide, aluminum oxide, beryllium oxide and boron sesquioxide.

10. In the process of preparing a cathode coating having the characteristic of self-sustained primary electron emission in vacuum, the step comprising heating a beryllium oxide coated cathode incarbon dioxide only and at a pressure of 50 mm. and a temperature of 900 C.

11. In the process of preparing a cathode coating having the characteristic of self-sustained primary electron emission in vacuum, the step comprising heating an aluminum oxide coated cathode in carbon dioxide only and at a pressure of 100 mm. and a temperature of 900 C.

References Cited in the tile of this patent UNITED STATES PATENTS 2,620,287 Bramley Dec. 2, 1952 2,873,218 Dobischeck et al Feb. 10, 1959 FOREIGN PATENTS 209,750 Switzerland July 16, 1940 511,706 Belgium June 14, 1952 

